1. Field of the Invention
The present invention generally relates to an automotive antiskid control system.
2. Related Art
An antiskid control system has been widely known. The antiskid control system can shorten the stopping distance of a vehicle by starting the adjustment of brake pressure on condition that the slip ratio of each wheel exceeds a target slip ratio (approximately 20%) in order to maintain the slip ratio of each wheel to the target slip ratio.
Among all of the conventional antiskid control systems Japanese Unexamined Patent Publication No. Hei. 5-16782, discloses that, when a vehicle is running on a road having a friction coefficient below a predetermined value (i.e., low .mu. road), the control characteristic with respect to the brake pressure of a wheel at low speed is established so as to weaken a wheel locking tendency as compared to a vehicle running over middle range of speeds. This is because the above system aims for exactly preventing the locking of the wheel so that the vehicle attitude and course can effectively be controlled by operating a steering wheel during the stopping period of time, in view of the fact that the time required to stop the vehicle on the low .mu. road becomes long.
Since vehicles employing spike-less snow tires are increasing in recent years, there are an increasing number of "mottled road surfaces" particularly around urban crossings where an ice part and a dry asphalt part alternatingly appear. If antiskid control is executed while a vehicle is running on the "mottled road surface," the slip ratio of a wheel increases when the wheel passes over the ice part as illustrated in FIG. 10. Therefore, the brake pressure is reduced and the deceleration of the wheel sharply falls. Subsequently, when the wheel passes over the dry asphalt part, the brake pressure is gradually increased and thereby the deceleration of the wheel rises. However, while the reduction of the brake pressure is sharply performed, the increase of the brake pressure is only gradually performed. Consequently, the wheel reaches the next ice part before the deceleration thereof sufficiently rises and then the brake pressure is sharply reduced again. For this reason, the stopping distance of the vehicle becomes longer compared with a case where the wheel is locked without executing the antiskid control. Particularly, the antiskid control according to the above prior art controls the brake pressure so that the locking tendency is weakened when the vehicle is running at low speed on the low .mu. road. Therefore, there is a problem that the deceleration of the vehicle further falls and thus the stopping distance thereof becomes considerably long.
On the other hand, seeing dry asphalt parts on the road surface, a driver expects a vehicle speed to be decelerated on the dry asphalt part. For this reason, the driver tends to depress a brake pedal stronger than usual on the "mottled road surface." However, as described above, the brake pressure is sharply reduced on the ice part according to the conventional antiskid control system, the deceleration is not realized so as to satisfy the driver's expectations but gives a sense of dissatisfaction to the driver.
As described above, most low .mu. roads are composed of the above "mottled road surface" in urban areas. Therefore, there is a problem that the conventional antiskid control system cannot satisfy the expectation of the driver with respect to the deceleration and cannot sufficiently shorten the stopping distance of the vehicle, either.